1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) panel, and more particularly, to an LCD panel that can obtain a picture image of high picture quality using a structure of a thin film transistor (TFT) having parasitic capacitance with small variation.
2. Discussion of the Related Art
FIG. 1 is an equivalent circuit illustrating a unit pixel of a general TFT-LCD array.
If a gate signal voltage is applied to the TFT-LCD array, the TFT is turned on so that a data voltage having picture image data is applied to a liquid crystal CLC through the TFT for a turn-on time of the TFT. At this time, current ION for charging the liquid crystal is obtained as follows.
                              I          ON                =                                            C              TOT                        ·            V                                τ            g                                              (        1        )            
In the equation (1), CTOT is a sum of a capacitor by the liquid crystal CLC and a storage capacitor CSTO for maintaining a phase of the liquid crystal until a signal is applied, V is the voltage and τg is turn-on time of a gate.
A total amount of charge in the capacitor by the liquid crystal and the storage capacitor is maintained until a next signal is received after the gate is turned off. Actually, since a leakage current IOFF exists due to resistance Roff of a channel layer of the TFT, distortion of a liquid crystal applying voltage VLC, i.e., a drop δVg (a difference between the maximum gate voltage applied when the gate is turned on and the minimum gate voltage applied when the gate is turned off) of the liquid crystal applying voltage occurs, thereby generating flickering. The leakage current IOFF can be expressed as follows.
                              I          OFF                =                                                            C                TOT                            ·              δ                        ⁢                                                  ⁢            Vg                                              τ              g                        ·                          N              g                                                          (        2        )            
In the equation (2), Ng is the number of total gates, and τg·Ng is time of one frame.
Especially, in a hydrogenated amorphous silicon (a-Si:H) TFT, overlap portions exist between a gate electrode g and a source electrode s and between a gate electrode g and a drain electrode d, so that parasitic capacitances of Cgs and Cgd exist respectively.
The parasitic capacitance is defined as the inductance or capacitance that has a function in addition to an original function of components according to the size, length, and arrangement of a device when the components are integrated to constitute a circuit. In this case, the parasitic capacitance means capacitance.
At this time, the parasitic capacitance Cgd generates voltage fluctuation ΔVp (a difference between the voltage charged to the pixel electrode when the gate is turned on and the voltage charged to the pixel electrode and voltage charged to the pixel electrode when the gate is turned off) in the liquid crystal applying voltage VLC by capacitive coupling when the TFT is turned off. Such voltage fluctuation temporally varies light transmissivity of an LCD panel and significantly acts on picture quality. The voltage fluctuation by capacitive coupling between the gate and the source/drain is compensated by a voltage Vcom applied to the common electrode. However, since the liquid crystal capacitance is a function of a data voltage, it is difficult to compensate all the data voltages. For this reason, flickering occurs.
The related art LCD panel has several problems.
If the gate is turned off, no variation of the voltage of the liquid crystal occurs due to the action of the capacitance Cst. Liquid crystal applying voltage drop occurs due to the capacitance Cgd in the overlap portion between the gate electrode g and the drain electrode d, thereby causing flickering when a picture image is displayed. Therefore, the amount of voltage drop depends on Cgd generated by the overlap area between the gate electrode and the drain electrode.
Generally, controlling an applying signal in a driving circuit can compensate such a voltage drop. However, if misalignment of the mask occurs in the process of fabrication, a voltage drop of the liquid crystal cannot be compensated by this method.